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Epoxy

Category: MaterialsCompiled from 4 source threads

Quotes are verbatim and attributed to their original authors.

Epoxy is the resin that wets out the fibreglass and bonds the whole layup into one structure. It cures by temperature as much as by time, and every formula has its own schedule — so the first rule on the forum is to work to your specific resin's data sheet.

It is driven by temperature

"This really depends on the specific epoxy you are using. There are so many formulas out there with different requirements and curing cycles. I would suggest that you try to contact either the seller or the manufacturer to find out if it cures at lower temps."
— chrismp · thread #5893, "epoxy cure time"

Warm the resin and the materials before you start — "the resin can be really thick at those cooler temps and hard to spread easily" (24Dave). A resin that "can be used without heat" still cures better and harder with some warmth.

Curing in the cold

Most builders work in unheated garages and barns, so a simple hot box is the common fix:

"If you have or get a heat gun, you can make a quick box perhaps out of a sheet of 1" panel insulation, a heat gun on about 240-260F will get the inside temp to about 130-140F in about 20 minutes, another 40 minutes at that temp will cure that brand of resin quite well. You can poke some oven thermometers through the foam box."
— 24Dave · thread #5893, "epoxy cure time"

Other proven boxes: a plywood enclosure lined with foil insulation and heated by four 100 W bulbs, or a fan heater inside a plastic tent (keep anything flammable away from the element). If you build in the cold regularly, a heat blanket and heated table remove the weather dependency entirely — see DIY heat blankets. For resin choice in the cold, "Sicomin 8500 and Greenpoxy are the easiest to work in the cold that I have tried" (Buckow).

Which epoxy

QCM (the EMV 0049 resin / ECA 032 hardener system) is named again and again for its value and its helpful technical support, and has its own expert Q&A thread (#614). Entropy Super Sap and West System also come up regularly. Whatever you choose, match the hardener to your working temperature and cure plan.

Epoxy versus other glues

A controlled forum test compared epoxy against polyurethane (Gorilla) glue and wood glue:

"The Poly-u sample flexed evenly like the epoxy sample but seemed to flex easier. The epoxy gave a nice firm even flex… Wood glue is useless. The fiber glass almost cracked when I flexed it."
— skidesmond · thread #3024, "Poly-U Glue vs Epoxy"

Polyurethane glue bonded firmly but expands four to five times as it cures; epoxy remains the standard for wetting out glass and giving a firm, even flex.

Source threads